This application is a continuation-in-part of my copending application for Printed Wiring Board, Ser. No. 364,309 filed Apr. 1, 1982, now U.S. Pat. No. 4,506,004 dated Mar. 19, 1985, and includes that application herein in its entirety by reference.
As set forth in my U.S. Pat. No. 4,424,089 issued Jan. 3, 1984, liquid photopolymer of paste-like consistency may be deposited to a desired thickness by screen printing. However, a loss of photo imaging resolution results for thicker layers. It is also shown therein that uniform thickness and very smooth surface of the polymer is desirable. Bubbles and surface screen marks are common flaws encountered when screening with liquid polymers. Also it has been found that conventional photopolymer coatings, such as those described in the literature and those presently available commercially, when deposited on a printed circuit wiring board over circuit traces of 0.002 in. (0.005 cm) thickness or greater with typical 0.012 in. (0.04 cm) width traces spaced by 0.012 in. spacings are subject to starvation or thinning of the coating over the traces, leading to unsatisfactory solder mask insulating properties. It is desirable to use a negative acting liquid photopolymer, which is low in cost and provides good solder coating characteristics.
Commercially available photopolymers of this nature, such as those identified in my U.S. Pat. No. 4,260,675 of Apr. 7, 1981, however, neither provide the proper viscosity nor resolution to produce high quality solder mask coatings of thicknesses up to 0.006 in. (0.015 cm) as set forth in my parent patent. For thick solder mask layers of high resolution therefore, it has been necessary to use expensive dry film such as taught by U.S. Pat. No. 4,413,051 to L. E. Thomas, dated Nov. 1, 1983.
The technique of transferring very thin layers of light sensitive, preferably positive acting, photoresist of several microns thickness from screens onto printed circuit boards is set forth in U.S. Pat. No. 4,376,815, M. J. Oddi, et al., of Mar. 15, 1983. The technique depends on a very thin layer of special photoresists containing volatile solvents which need be heat dried to remove solvents before photo imaging. Also because of very thin viscosity, even with tight mesh screens, these resists require special techniques such as misregistration of successive printed wiring boards to keep from dripping off drops of liquid resist in the presence of through holes in the printed wiring boards. Also because of these very low viscosities, thicker film layers such as 0.002 in. (0.005 cm) to 0.006 in. (0.015 cm) could not be screen deposited. Furthermore, thin viscosity liquid resists run off the wiring traces to produce inadequate solder mask thickness or starvation over closely spaced narrow printed wiring traces. This particular photoresist need be altered for flow characteristics by increasing the percentage of solids to solvent or by the addition of fillers so that the photoresist does not run through the very fine mesh screen or run off the smooth surface of the substrate while heat drying to remove the solvent. The diazo photo resists used are not suitable for photo imaging in the liquid state with solvents present, and thus the interspersed heat drying cycle makes this technique unsuitable economically for competitive printed wiring board usage to compete with thicker dry film layers and better photo imaging resolutions such as available for use as permanent solder mask coatings photo developed in-situ on printed wiring boards, even if were known how thicker layers of the solvent containing resist could be processed and could be photo imaged with high resolution.
To attain thick coatings of photo resists such as 0.005 in. (0.013 cm) is a problem in the art as shown in U.S. Pat. No. 4,413,051, supra, particularly when high photo imaging resolution is required. Thus, it is not known in the art before my invention how to build up liquid photoresists capable of remaining in-situ without a processing step before photo imaging when thicknesses of more than about 0.0005 in. (0.0013 cm) are required, nor is it known how to obtain high photo resolution from thick liquid photo resist layers.
The use of liquid photo polymers to attain thicker layers such as 0.003 in. (0.008 cm) by an intermediate exposure step to polymerize the liquid polymer layer in place on a printed circuit board before photo imaging is set forth in U.S. Pat. No. 4,291,118, P. Boduch et al., of Sept. 22, 1981. Thus, an intermediate actinic radiation exposure step is necessary in order to hold the liquid photopolymer layer in place, and a difficult air knife spreading step for the liquid polymer is required during radiation results in a costly process and costly tooling. The system becomes critical and resolution diminishes when a polymer is partially prepolymerized. Even so this process cannot be used on boards with circuit traces to produce minimum thickness coatings over circuit traces.
As seen from British Patent Specification No. 741,470, E. I. Du Pont, published Dec. 7, 1955, it is conventional in the liquid photopolymers that to attain thicker layers, the intermediate photo polymerization step is required before photo imaging, or alternatively, the liquid has added fillers of silicon or the like, which can diffract actinic light and therefore disturb resolution. Problems are encountered in keeping the liquid layer flat and constant in thickness during polymerization. These techniques are developed for the purpose of obtaining half tone images and very poor resolution is attained, because of diverging radiation in the photopolymer to form conical sides rather than sharp straight sides required in photo imaging of solder masks onto modern high resolution printed wiring boards.
In my parent application, the thickness of about 0.006 in. (0.015 cm) is attained by superimposing two thinner layers of liquid photopolymer of paste-like consistency each deposited from a screen to control thickness. The objective of bubble free adhering contact at the substrate and inner layer surfaces and a smooth bubble free outer surface is necessary to produce high resolution photo images. However, this objective has been found to be critically related not only to critical photo qualities of the polymer for attaining high resolution photo imaging, but also to the viscosity of the photopolymer. No available photopolymers of satisfactory viscosity and photo qualities existed in the published arts or available commercial polymers for high resolution photo imaging of permanent solder mask coatings directly on liquid photopolymer coatings placed on the substrate. Thus it was found that the available liquid negative acting photopolymers were either very thin for the production of thin layers or very thick and containing fillers or additives which could refract or diffuse actinic light and thus limit resolution and/or curing depth, particularly in layers as thick as 0.004 in. (0.01 cm) to 0.006 in. ( 0.015 cm).
It is an object of this invention to overcome the deficiencies of this prior art and to provide improved liquid photopolymer solder mask resists for producing with screen depositing through coarse screens smooth bubble free surfaces and high photo imaging resolution throughout thickness ranges of the order of 0.002 in. (0.005 cm) to 0.006 in. (0.015 cm). Thus, in situ coatings are desired to photo develop for obtaining high resolution patterns permanently covering printed wiring traces on printed wiring boards. The liquid polymer coatings need be photo imagable in place in liquid polymer form and controllable by screen printing to attain desired layer thicknesses and proper flow characteristics over rough substrate terrain afforded by wiring traces thereon of thicknesses up to about 0.005 in. (0.013 cm).